Boot (Trunk, Cargo area) struts very stiff if door left open.

I noticed this too when we went camping this year - there were times when the boot was open for quite a while, and it was noticeably harder to close when it was left like that. We were in France and the weather was very warm (also about 30°C), so maybe it is temperature related too.

Once you close it again, it then works normally - so it's probably just something to do with the internal piston pressures "equalising" when it's left open.

 

...actually thinking about it it is probably to do with pressure and heat of the gas (first law of thermodynamics). When closed, the gas is compressed, and actually the struts will heat up during closing due to the increased pressure of the gas inside. This heat gradually dissipates until it reaches equilibrium again.

When allowed to expand (as it's still under pressure), the pressure will drop and the gas will become cooler, and start to absorb heat from the environment (which will then increase the pressure again). So when you come to close it again, the pressure in the cylinder has increased over what it was when it was first opened, due to the expanded gas having had more time to absorb heat from the environment and reach a higher temperature, and therefore higher pressure.